Non-invasive imaging and analysis of targets using optical coherence tomography (OCT) is a powerful technique for acquiring sub-surface information embedded in targets without damaging the target or system being analyzed.
The embedded information in a particular sheet of paper of base document can be imaged and analyzed by Optical coherence tomography (OCT), a technology for non-invasive imaging and analysis. There are more than one OCT techniques. Time Domain OCT (TD-OCT) typically uses a broadband optical source with a short coherence length, such as a super-luminescent diode (SLD), to probe and analyze or image a target.
Multiple Reference OCT (MRO) is a version of TD-OCT that uses multiple reference signals. Another OCT technique is Fourier Domain OCT (FD-OCT). A version of Fourier Domain OCT, called Swept Source OCT (SS-OCT), typically uses a narrow band laser optical source whose frequency (or wavelength) is swept (or varied) over a broad wavelength range. In TD-OCT systems the bandwidth of the broadband optical source determines the depth resolution. In SS-OCT systems the wavelength range over which the optical source is swept determines the depth resolution.
Another version of Fourier Domain OCT, often referred to as Spectral Domain OCT (SD-OCT), typically uses a broad band optical source and a spectrometer to separate out wavelengths and detect signals at different wavelengths by means of a multi-segment detector.
OCT depth scans can provide useful sub-surface information including, but not limited to: sub-surface images of regions of targets; measurement of thickness of layers of targets. More generally OCT depth scans can provide useful sub-surface information regarding attributes of targets.
Documents, such as bank notes and legal documents require security features to protect against counterfeit documents. There is an on-going need for improved protection of valuable documents against counterfeiting. The ability of OCT to analyze information embedded within a target enables adding a security layer to documents by embedding information or data during the manufacturing process of the paper (or base document).